In a NAND flash memory, an isolation system of an STI (Shallow Trench Isolation) structure is used. A silicon oxide film is used as the STI structure. A Yupin/Enda effect that a current leaks between floating gates adjacent to each other through the silicon oxide film of the STI is increased with a progress of microfabrication of an element pattern. Due to the leak current, a malfunction is generated in read/write of the memory and becomes prominent in generations from a generation of a 30-nm element size. In order to reduce the Yupin/Enda effect, the leak current in the STI is decreased by selecting various materials for the STI or proposing the structure.
For example, a method in which a cavity structure (air gap) is used in the STI can be cited in order to solve the problem. The air gap has an excellent insulating characteristic because air exists in the air gap. Additionally, because the air gap has permittivity of 1, the leak current can be decreased to the utmost limit.
However, various problems are generated in forming the air gap. For example, there is a method, in which the air gap is formed by depositing a film having a bad burying characteristic in a trench by a CVD (Chemical Vapor Deposition) method such that the trench is not filled. However, in this method, adhesion of the film to a sidewall of the trench adversely affects a charge retaining characteristic of the memory. The insulating characteristic is insufficiently obtained because of the insufficient formation of the air gap.
On the other hand, there is also proposed a method in which, after the trench is completely filled with a sacrifice film and an interelectrode insulating film is formed on the sacrifice film, the sacrifice film is partially exposed and the sacrifice film is removed by etching. A PSZ (polysilazane) film is used as the sacrifice film. However, unfortunately a large cavity serving as the air gap is hardly formed because the PSZ film has a relatively high etching resistance characteristic.